This invention relates to a method of treating a semiconductor body.
A charge-coupled device (CCD) may be made by processing a silicon wafer of p conductivity using conventional MOS technology to form a plurality of buried channels of n conductivity beneath the front surface of the wafer (the surface through which the wafer is processed). Each channel is made up of a linear array of like elementary zones. A clocking electrode structure overlies the front surface of the wafer, and by application of selected potentials to gate electrodes that form the clocking electrode structure, charge present in a given elementary zone of a channel may be advanced through the linear array of elementary zones, in the manner of a shift register, and extracted from the channel. Charge may be generated in the channels photoelectrically. Thus, if electromagnetic radiation enters the wafer it may cause generation of conduction electrons and these conduction electrons may become confined in one of the elementary zones.
A CCD may be used to generate an electrical signal representative of the distribution of light intensity over the channel region of the CCD. In such an imaging CCD, it is desirable that the thickness of the die be not much greater than the depth of the interface between the channels and the substrate. This may be achieved by securing the wafer at its front surface to a reinforcing member, and thinning the wafer from its back surface. The terminals of the electrode structure are covered by the reinforcing member. Electrical contact is made to the terminals of the electrode structure via conductor runs that extend from the terminals to bond pads on an exposed surface region on the reinforcing member. A method of providing such conductor runs is described in co-pending patent application Ser. No. 07/114,884 filed Oct. 29, 1987.
In operation of a CCD, the substrate is typically held at a ground potential level, and the gate electrodes of the clocking electrode structure are clocked between voltages of approximately -5 to +15 volts relative to ground. In order to hold the substrate at ground, a substrate contact is formed on the front surface of the wafer, outside the channel region. In order to provide an ohmic contact between the substrate contact and the substrate, boron may be implanted into the substrate where the substrate contact is to be formed. The substrate contact may be in multiple segments about the periphery of the channel region, with each segment connected to a bond pad on the reinforcing member in similar fashion to that in which connections are made to the terminals of the electrode structure. The various segments of the substrate contact are spaced from one another in order to permit conductor runs to pass from the terminals of the electrode structure to their bond pads.
It has been found that defects in the wafer are associated with the substrate contact. As the wafer is thinned by etching, pits appear over the substrate contact and lead to a more rapid etching of the silicon in those regions than elsewhere. Also, relatively large chunks of silicon frequently lift out of the substrate over the substrate contact, exposing the contact metal and reflow glass deposited over the front surface of the wafer. These materials etch rapidly in the etchants used to thin the wafer, severely undercutting the CCD and rendering it unusable.
It is known to deposit metal over the back surface of the channel region of a backside illuminated imaging CCD and to remove portions of the metal by etching, while leaving metal over other portions of the back surface. This metal is provided to shield selected areas of the channel region from illumination and does not serve an electrical purpose in connection with operation of the CCD.